Contact lenses are widely used for many different types of vision deficiencies. These include defects such as near-sightedness and far-sightedness (myopia and hypermetropia, respectively), and defects in near range vision usually associated with aging (presbyopia). Current opinion holds that presbyopia occurs as a person ages when the lens of eye begins to crystallize and lose its elasticity, eventually resulting in the eye losing the ability to focus on nearby objects.
Some presbyopic persons have both near vision and far vision defects, requiring bifocal lenses to properly correct their vision. Many people prefer wearing contact lenses to correct their vision rather than bifocal eye glasses.
A typical single vision contact lens has a focus, which is the point on which parallel rays of light focus when the lens is placed perpendicular to the parallel rays, and an optical axis, which is an imaginary line drawn from the focus to the center of the lens. A posterior surface fits against the cornea and an opposite anterior surface has a vision surface that focuses light to correct the eye's vision. In the case of a typical spherical lens, the vision surface has a single radius of curvature that is the distance from any point on the vision surface to a point on the optical axis referred to as the center of curvature. A bifocal lens has at least two vision surfaces on the anterior surface of the lens: a distance vision surface, for gazing at far off objects, and a near vision surface, for gazing at close objects (e.g., while reading).
Effective use of a bifocal contact lens requires translation of the eye between vision surfaces when the eye changes from gazing at an object at a distance to gazing at a nearby object. In such a situation, the pupil must move from being subtended by the distance vision surface to being subtended by the near vision surface.
In designing a lens, translation is of particular importance. Most lenses have difficulty translating across the surface of the eye when the visual direction of the eye changes from horizontal gaze distance vision to down gaze near vision. This is due to the ability of a soft contact lens to conform closely to the shape of the cornea. For this reason, soft translating bifocal contact lenses are uncommon. Thus, users who desire bifocal contact lenses are usually limited to using the more uncomfortable hard lenses, while those who wish to wear soft contact lenses are usually limited to wearing mono-focal lenses.
Therefore, there is a need for a soft bifocal contact lens that supports translation across the surface of the eye when the eye changes position from distance vision to near vision. Lenses with a translation “ridge” on the front surface have been produced; however, if the ridge feature does not have an undercut, the lid may not engage the lens and hence, the lens translation amplitude may be insufficient, resulting in poor bifocal performance.
If the ridge is located close to the edge, on a steeper position of the lens, the lid tends to roll over the ridge feature and the lens does not translate. If the ridge is positioned nearer to the center of the lid, where the lid can engage the ridge, the lower lid may not provide enough movement for effective amplitude of the lens translation.
An undercut features currently require slides in the mold bases during the injection molding process, which prevents use of high volume mold manufacture.